An extrusion die is used to extrude molten thermoplastic material into a relatively thin film or sheet. Conventional extrusion dies have a die cavity with the general shape of a coat hanger. These extrusion dies have been generally referred to as coat hanger dies. Typical coat hanger dies include an inlet, an inlet manifold, a generally triangular shaped preland channel, a final land channel, and a die exit or gap. In some embodiments, a back edge of the preland channel includes linear edges that form a taper converging towards a die entrance. In alternate embodiments, the taper converges away from (or diverges towards) the die entrance. The preland channel provides a resistance to flow that varies over the width of the die to uniformly spread the thermoplastic material across the entire die.
The rheological characteristics of the molten thermoplastic materials and the pressure these thermoplastic materials exert on the die bodies sometimes result in clamshelling, or non-uniform deflection. Clamshelling results in uneven flow of the thermoplastic material through the die exit. These or other nonuniform patterns are undesirable for the production of flat sheet materials. The nonuniformities can be corrected by adjusting the height of the die exit. Most extrusion dies are equipped with some form of die lip adjustment mechanism to adjust the height of the die exit. If a particular die is used to form a large quantity of a single layer of material, and the extrusion process is not subject to any interruptions, then the adjustment of the height of the die exit is an acceptable method of removing or at least addressing these nonuniformities. However, since the performance of an extrusion die is influenced by a number of factors including flow rate, temperature, the nature of the thermoplastic materials, and the like, the use of the lip adjustment mechanism to remove or minimize these nonuniformities may not always be practical where a die is to be used for short runs to produce different products. For each production change, the die lips may be adjusted to the new conditions. This results in a loss of production time and a waste of material as off-specification product is produced during start up.
When extruding thermoplastics using a conventional die, the molten polymer exits the die and travels through an air gap prior to contacting the surface of a roller receiving the extrudate. As is well known in the art, as the extrudate flows across the air gap and is pulled away from the die by the rotating roller, stress can be induced in the web. Additionally, variations in the web's gauge (or thickness) can occur, particularly at the edges which become thicker as the web narrows. There is little that can be done to control the flow of the extrudate in the air gap. Furthermore, die lines are formed if the extrudate drags along one of the die lips. These and other nonuniformities in the extrudate can be exacerbated by the air gap between the die exit and the surface of the roller receiving the extrudate. Such defects are unacceptable in most extruded products, but are particularly troublesome in products that require good optical properties. Accordingly, die builders are asked to minimize the air gap.
In the past, die builders have typically attempted to decrease the air gap by angling the die such that the die exit is positioned as close to the roller as possible. However, this approach weakens the die and mechanical limitations become challenging. Other proposals to address these problems have included the design of different manifold configurations. In one such design, the back line of the preland portion is structured to be a uniform distance from the die exit. While this particular design may minimize the above problems, there are concerns relative to maintaining the flow through the die without creating areas of substantially higher residence time, which over time could lead to degradation of the thermoplastic material.
Accordingly, there exists a need for an extrusion device with essentially no air gap for extruding low stress sheet and film with acceptable optical properties, minimal die lines, and minimal edge bead formation without compromising the mechanical integrity of the die.